1. Field
This invention relates to ceramic coating and bonding methods, and more particularly relates to fusion-formed, ceramic coating and bonding methods with uniform ceramic metallizing compositions and specially graded, microscopically substantially perfectly defect-free bonded regions to produce reproducibly strong and thermomechanically shock-resistant ceramic coatings or bonds.
"Ceramic" means not only the usual ceramics such as alumina, zirconia, beryllia, mullite, cordierite, silicon carbide; but also quartz, intermetallics, diamond, boron, graphite, carbon, silicon, and various other carbides, nitrides, aluminides, or borides, glasses, machinable glasses, Corning's Vision glass; but also the surface of many reactive metals such as aluminum, magnesium, chromium, silicon, titanium, or zirconium which always have oxides, nitrides, hydrides, or other compounds of reactions of the metal with the environment.
2. Prior Art
Various methods have been developed to coat ceramic or metal with, or to join metal to, ceramics. But none gives inexpensive, stable, strong, and temperature resistant products. Reliable ceramic coatings or joints are not commercially available worldwide at any cost, even for small joint sizes.
Under a well-coordinated intensive effort on ceramic-metal bonding, Japan has been the most successful country in the development and commercialization of products involving metal-ceramic bonds. They already have successfully: (1) used a ceramic turbocharger (NGK, Nissan), (2) produced an all ceramic swirl chamber for diesel engines (Mazda, NGK), and (3) prototyped a ceramic turbomolecular pump (Mitsubuishi and Japan Atomic Energy Research Institute). But according to Prof. T. Suga of the University of Tokyo in his 1989 review paper on the "Future Outlook in Japan" (Copy enclosed), the practical useful temperature of the best Japanese ceramic joints to special "matching" metal alloys is only 600.degree. C. Further, the bond strength decreases rapidly with temperature, because the reaction products in their bonded regions become weak and brittle under thermal stresses. They consider the improvement of the thermomechanical shock resistance of their brazed ceramic joints to be an urgent task.
The European effort, mainly in Germany and France, has been even less successful. Germany failed to reach their goal after the first ten-year (1974-1983) program and its follow-up in 1983-1986. Their present program (1985-1994) merely emphasizes on reproducible mechanical properties and component reliability. The US Department of Energy supports much of US ceramic joining R&D. It also had to renew annually the ceramic automotive program after 10-year, 50-million intensive work, mainly producing a specification for automative ceramic-metal joints.
Each metal-ceramic joint or bond must be specially designed. The factors in joint design include metal and ceramic composition, joint failure modes, parts shapes and sizes, thermal and other demands. The requirements for the National Aero-Space Plane (NASP) is totally different from those of the diamond heat sinks or fusion reactors. A ceramic-metal bond designed for maximum mechanical strength is usually not the best for thermal conductances, which is critical in heat sinks. What is best for one application (e.g., for preventing rapid heating failures) may even be precisely the worst for another (e.g., for preventing severe quenching failures), as shown by the functional grading technique described in this application. On the NASP, for example, the best titanium-Si.sub.3 N.sub.4 joint for the turbine subjected to rapid heating should not be used for the wings of the same plane subjected to possible ice quenching failures. A joining method for many conditions may not be the best for any application.
Different physical, chemical, and electrical metallizing or film-forming methods have been developed for metal-ceramic bonding. Each method has its unique advantages. Some, for example, are atomically precise. Others thoroughly clean the substrate surfaces for better adhesion. Some others result in crystalline epitaxy, which is necessary for semiconductor devices. Still others produce splat cooling and superfine grains, with resultant enhanced mechanical properties, for example, increased Young's modulus. Still others are done at low temperatures to avoid unwanted thermal effects. But none deal effectively with the many critical problem to be addressed in this invention.
Most ceramic-metal joints have bonding regions that are not microscopically perfect or 100% dense, severely damaging the joint mechanical strength and thermal or electrical conductivities. Sintered, solid-state formed, hot or cold pressed, diffusion bonded, or even liquid infiltrated bonding layers cannot be fully dense, no matter how high the vacuum, external pressure, or processing temperature. This is because trapped gases cannot be compressed to zero volume, particularly if they are sealed off by initial densifications. Evaporated, sputtered, plasma, and electrolytic or electroless deposits generally are packed plates. Packed particles can never be 100% dense. The maximum density in packed spheres is only about 74% for the idealized close-packed, face-centered or hexagonal packing structure. Ceramic metallizing with mixed W/Mo and Fe/Mn powders have voids and segregations initially already present in the coated layers. These defects, generally remain after high-temperature processing, because of contamination, inadequate melting and fluxing, and diffusion voids, and other chemical reactions. Repeated metallizing, sintering, nickeling, as suggested by, e.g., the U.S. Pat. No. 3,901,772, do not solve the basic problems.
Achieving full density in chemical vapor deposition (CVD) or physical vapor deposition (PVD) requires not only complete absence of dust, contamination, inclusion, and trapped gas, but also special ambient such as excellent vacuum, not gaseous ambient under atmospheric pressure. Deposited films also require perfect cleaning and optimal nucleation and crystal growth. Nucleation and crystal growth is still not a science. The later, in particular, requires unknown but continuously varying growth rates and temperature profiles. After billions of dollars of CVD work (e.g., in electronics), defects in CVD films are still prevalent. Pores, for example, often reach up to 10 or 20% in even the most studied diamond films, according to a 1990 DTIC report referred to elsewhere. This is so regardless of whether high or low-pressure, high or low-temperature, plasma or laser enhanced or not, or the type of equipment, carrier gases or reactants used. Unless ultra-high vacuum is used, active metal bonding methods employing Ti, Zr, Nb, Cr, . . . always contains surface oxides, nitrides, carbides, . . . which lead to pores or cracks (from mismatch between, e.g., oxide or metal) and refractory, non-wetted or non-bonded areas.
The metal powders used in the common ceramic metallizing processes are limited to 325 or 400 mesh sizes, or still tens of microns in diameters. Finer powders are costly, and generally surface contaminated. These fine mixed powders are always segregated, and cannot produce thin metallized layers one micron or 100 Angstroms (A) thick, nor with thickness accuracies of less than 1,000 or 100 A.
Hence, most ceramic-metal joints are not substantially perfectly bonded, not only microscopically but even macroscopically. By "macroscopically or microscopically substantially perfect wetting or bonding", it is meant that no defects are visible in the form of voids, cracks, excessive fluxes, non-wetted, or non-bonded areas under the microscope or on microphoto at 3-20 or 100-1,000 times magnification, respectively. Microphotos such as those in FIGS. 2 and 3 (at 1,000.times.magnification) of the Li's "Diamond Metallization" paper given in Ref. E mentioned elsewhere show microscopically perfect bonding with none of the defects mentioned above. These microphotos are available to the public since 1990 via the SDIO Final Report, Ref. 17, in the "Diamond Metallization" paper.
Many problems still exist with present ceramic metallizing, coating, and bonding methods. A serious problem is the instability and unreliability of even the best ceramic-metal bonds made in, e.g., Japan, as mentioned above. Another problem is the difficulty of achieving on the ceramic surface uniform metallized layers, or even coated layer of the metallizing powders.
Take, for example, the commonly used heavy metal processes, such as W-Yttria (W--Y.sub.2 O.sub.3), W--Fe, or Mo--Mn. In these and many similar methods, segregation of the mixed metal powders takes place due to their differing specific gravities, shapes, sizes, porosities, and surface smoothness. These segregation occur at all times: during the mixing of the powders, storing of the powder suspensions, application of the suspensions, settling of the suspended powders in the applied coatings, and drying of the applied coatings. Further, these segregations occur so fast as to be practically uncontrollable, as will be shown shortly.
In general, spherical, heavy, large, smooth, and dense powders settle first and early in the binder or suspension medium. Upon settling, these powders tend to roll or move sidewise or downward toward the corners or boundaries faster and further than odd-shaped, light, small, rough, and porous powders of otherwise identical characteristics.
Take the W--Y.sub.2 O.sub.3 mixed powders in an organic binder of nitrocellulose in butyl carbitol acetate with specific gravities of 19.3, 4.5, and 0.98, respectively. Such a suspension, even if perfectly mixed up by shaking, stirring, roller-milling, or otherwise, will immediately tend to segregate. More specifically, the initial settling acceleration due to gravitational minus buoyancy forces on W powders is 980.6.times.(19.3-0.98)/19.3=930.8 cm.times.cm/sec, while that of Y.sub.2 O.sub.3 powders is only 767.0 cm.times.cm/sec.
In a mixing, storing, or carrying bottle 10 cm high and containing a perfectly mixed suspension of these mixed metallizing powders, the time for the W powders to completely settle out is only 147 ms (milliseconds), if uniform acceleration is assumed. At the tip of a paint brush having a suspension drop 0.3 cm in diameter, the complete settling time of these W powders is merely 25.4 ms, while on a horizontally painted or sprayed layer 0.1 cm thick, the same settling time is only 14.7 ms. In all these cases, the complete settling time for the Y.sub.2 O.sub.3 powders is always the square root of 930.8/767.0=1.21, or 21% longer, as shown in the U.S. Pat. No. 4,890,783.
Assuming uniform accelerations, mixed powder segregations may be completed within 147 to 14.7 ms. Such short times indicate that the W--Y.sub.2 O.sub.3 powder segregations are beyond human controls. Painted or sprayed mixed powder layers are thus always not uniform.
In metallizing onto a horizontal ceramic surface, most of the W powders immediately settle out. The first coated layers are therefore always very rich in W (melting point 3,410.degree. C.), and correspondingly very poor in Y.sub.2 O.sub.3. These first layers are too refractory for the preset metallizing temperature (up to about 1550.degree. C.) to melt, so that the ceramic surfaces are not sufficiently metallized, or not at all. The last settling layers, on the other hand, are too rich in the fluxing Y.sub.2 O.sub.3. Thus, the ceramic surfaces are improperly metallized, with bottoms undermetallized layer and top glassy layer. The metallized bonding layer is either erratic, or very weak in strength and thermal or thermal shock resistance.
Hence, common W/Mo metallization on ceramics generally produces unreliable or uncontrollable results. The metallized surface often contain loose and unmetallized spots with high heavy refractory metal content, or non-wettable spots due to the high flux content. Even repeated metallization, brushings, and nickel or copper platings, as suggested in U.S. Pat. No. 3,910,772, do not solve the basic microstructural problem due to powder segregation. The entire process is costly, critical and involved, and yet nonuniform. The resultant ceramic-metal joints or ceramic coatings on metals are also weak, costly, nonreproducible, and usually not vacuum-tight, or temperature-resistant, e.g., less than 600.degree. C. even in the best Japanese joints with superfine ceramics and "matching" high-nickel metals, as mentioned above.
Painting or spraying onto vertical or inclined surfaces results in additional segregations and gradations, and gives added poor uniformity, reproducibility, and bonding results. While only the effect of gravitational density segregation has been considered in some detail in the U.S. Pat. No. 4,890,783, the other segregation variables such as powder shape, size, porosity, and surface roughness are also important. This and other previous inventions achieve significant improvements never before possible. Still, absolute joint perfection is evidently impossible.
A second important problem with common joining processes is the lack of control, or even understanding, of dynamic mismatches of temperatures, coefficients of thermal expansion (CTE's), stresses, and strain profiles in the joint region, and their variations with time. Another aspect of this invention is therefore to describe such dynamic mismatch phenomena, and to specially tailor-grade the composition and/or physical property profiles of the joint region so that the maximum or critical transient mismatch stresses never exceed the local material strength at any point inside the joint region, at any time during the heating or cooling of such joints in processing or service.
A third problem results from our incomplete understanding of the required microstructural, chemical, and physical properties of the interfacial regions in the ceramic-metal joints.
These gravitational segregation, dynamic mismatch, and joint design problems have been described and preliminarily solved in the U.S. Pat. No. 4,890,783 and other patents. This invention is to continue to improve upon these previous solutions.
Accordingly, an object of this invention is to provide improved ceramic-metal joints and joining methods;
A further object of this invention is to provide improved ceramic metallizing methods for these joints; PA1 A broad object of this invention is to minimize gravitational segregations of the components in the metallizing methods during or prior to the joining; PA1 Another broad object of the invention is to functionally tailor-grade, both parallel to and normally of the thin bonding region, the composition and property profiles in the bonding regions to ensure that the maximum dynamic or transient stresses do not exceed the local material strengths at any point and time; PA1 A further object of the invention is to provide a specially microengineered, highly wetting and perfectly bonded interfacial bonding layer of the optimum characteristics to achieve defect-free, tough, and very strong joints; PA1 A still further object of the invention is to provide uniformly thin (1 micron, 1,000 A, or 100 A) bonding layers with controlled uniformity and thickness accurate to 100 or 10 A; PA1 Another object of the invention is to flawlessly coat metals or ceramics with protective materials, particularly to produce tough, strong, thermochemically stable, and thermomechanically shock-resistant composites; PA1 Another object of the invention is to provide improved method for making, and products of, diamond, silicon carbide, and other ceramic joints to metals; PA1 Further objects and advantages of my invention will appears as the specification proceeds.